Prevention or reduction of scaling on a heater element of a water heater

ABSTRACT

The invention provides a method of using a water heater arranged for heating an aqueous liquid wherein the water heater comprises a heating element to heat aqueous liquid in the water heater. The method comprises (a) heating aqueous liquid in the water heater with the heating element), wherein the heating element is in contact with the aqueous liquid; and (b) applying a first AC voltage between the heating element and a counter electrode, and applying a DC voltage between the heating element and the counter electrode, wherein the DC voltage is at least 0.5 V, and wherein the heating element is chosen as positive electrode.

FIELD OF THE INVENTION

The invention relates to a method of using a water heater and to a waterheater arrangement that can be used for such method. The invention alsorelates to an electronic device comprising such water heaterarrangement.

BACKGROUND OF THE INVENTION

Water heating devices are applied in all kind of applications, such assteam irons, electric kettles, hot drink vending machines, etc. Aproblem with such devices is that scale may form on the heating elementsthat are in contact with the water.

During operation of for instance a steam generation device, water issupplied to a part of the water infrastructure where it is heated, suchas in the (external) boiler of a system iron, as a consequence of whichscale may be formed. If the scale is not (periodically) removed cloggingup may occur, as a result of which the performance of the steamgeneration device may decrease and, eventually, the steam generationdevice may not be fit to be used anymore.

WO2007007241 for instance describes a steam ironing system comprising asteam iron and a boiler system having a boiler for generating steam,wherein the steam iron and the boiler are connected to each otherthrough a steam hose. During operation of the boiler system, scale isformed in the boiler. For the purpose of removing the scale from theboiler, a rinsing process is performed on the boiler system at regularintervals. During the rinsing process, a rinse valve connected to awater outlet positioned at a bottom of the boiler is opened, and wateris discharged from the boiler to a rinse container. In the process,scale particles are carried along with the flow of water. Preferably,pressure is built up inside the boiler prior to opening the rinse valve,so that the water is forcibly ejected from the boiler, whereby theeffectiveness of the rinsing process is enhanced. During the rinsingprocess or at the end of the rinsing process, scale solvent may beintroduced into the boiler.

US 2003192436 describes a method for the generation of steam, inparticular for a cooking device, whereby a fluid within a steamgenerator container is brought to the boil by the heating of at leastone heatable wall surface of the steam generator container. Said fluidis set in rotation by the heating and thus pressed against the heatablewall surface, due to centrifugal forces arising from the rotation. Thesteam generated as a result of the evaporation of at least a part of thefluid escapes from the steam generator container through a steam outletand droplets of fluid entrained in the steam are separated out by atleast one first rotor, rotatably mounted in the steam generatorcontainer, whereby the fluid is set in rotation by said rotor. Theinvention further relates to a device using the above method.

US2001018866 describes an arrangement for indicating the calcificationstatus of flow heaters, particularly in espresso machines, with the flowheater having a cold-water inlet line and a hot-water outflow line, andwhich is intended to precisely indicate the calcification status, yethave an uncomplicated design. For this purpose, a differential-pressurefluid gauge is provided, with the gauge having two pressure chambersseparated by a diaphragm, with one of the pressure chambers being in afluid-conducting connection with the cold-water inlet line and the otherpressure chamber being in a fluid-conducting connection with thehot-water outflow. The diaphragm acted upon by the pressure differencebetween the two chambers is coupled to indicator elements.

Hard water containing significant amount of Ca²⁺ and HCO₃ ⁻(bicarbonate) may form scale (CaCO₃) upon temperature increase via thefollowing chemical reaction:Ca(HCO₃)₂→CaCO₃+H₂O+CO₂

Especially boiling water will separate scale, the scale will form in thewater but also on the heating element itself as it has the highesttemperature. In time scale will grow on the heating element and wheninternal stresses increase it will break loose from the element. Severaltreatments of water to prevent scaling have been claimed in theliterature. A well known method is the use of ion exchangers were Ca²⁺is exchanged for Na⁺ or H⁺. A second well known method is the use ofphosphonate that in small amount is added to the water and inhibits theformation of seeding crystals in the hard water, effectively preventingthe growing of crystals and thus the formation of scale (see also thecited art above).

In the former a cartridge needs to be used with the ion exchange resininside. After depletion the cartridge needs to be regenerated orreplaced with a new one. In the latter case continuously phosphonateneeds to be added as the phosphonates have limited stability at pH7-8.5, the pH of hard water. The continuous addition can be implementedfor example by using a hard-pressed tablet that very slowly liberatesthe phosphonates into the water. This way of working has been used inprior art steam irons. However, chemicals are added into the water whichmay be a drawback, for instance when water is (also) meant to bepotable.

Physical methods to prevent scale formation have also been claimed butthese may have a less clear working principle and the efficacy maysometimes even be doubtful in some cases. For example the use of(electro) magnets placed on water tubing for scale prevention is anexample of a poorly understood and non-repeatable scale preventionmethod.

SUMMARY OF THE INVENTION

Hence, it is an aspect of the invention to provide an alternative methodto prevent or reduce scaling in a water heater and/or an alternativewater heater arrangement, which preferably prevents or at least partlyobviates one or more of above-described drawbacks and/or relatively morecomplicated constructions or solutions of the prior art. It isespecially an aim of the invention to prevent or reduce the formation ofscale on heating elements (such as a heatable wall or an immersionheater) in heating appliances and/or to decalcify calcified surfaces ofsuch heating elements.

Here, electrochemical scale prevention and/or removal from an aqueousliquid such as water is proposed. A principle could be to have twoelectrodes in the water connected with a DC power supply. At the anode(+electrode) oxidation is taking place. At the cathode (− electrode)reduction is taking place; in practice this means that at the cathodewater is reduced:2H₂O+O₂+4e ⁻→4OH⁻

The formation of OH⁻ will increase pH locally and transform the HCO₃ ⁻into CO₃ ⁻. The CO₃ ⁻ will react with the Ca²⁺ and calc will precipitateon the cathode.

At the anode oxidation takes place. When the anode material is oxidationresistant then water is oxidized towards oxygen and acid. The acid willdissolve calc that has been deposited on the electrode and the electrodewill remain clean when used in heated (hard) water:2H₂O→O₂+4H⁺+4e ⁻

When the anode is reactive it may be oxidized. For example metal anodeswill dissolve unless very stable metal (Pt), certain transition metaloxides or carbon anodes are used. Calcified steel can be decalcified byapplying a positive voltage but its effect is limited by the corrosionresistance of the metal making only small voltages/currents feasible.

In summary, such simple setup may remove scale from the water bydepositing it on a cathode and keeping the (oxidation resistant) anodeclean. A drawback, besides the need for corrosion resistant anodematerial, may be that the cathode needs to be cleaned at regularintervals. Hence, the invention especially provides solutions that mayalso solve this problem. Below, some preferred embodiments are describedin more detail.

It has surprisingly been found that a method wherein an AC signal and aDC signal are applied, scaling can be reduced, prevented or removed.Hence, the invention especially provides a method of using a waterheater arranged for heating an aqueous liquid wherein the water heatercomprises a heating element to heat aqueous liquid in the water heater,the method comprising:

-   a. heating aqueous liquid in the water heater with the heating    element, wherein the heating element is in contact with the aqueous    liquid; and-   b. applying a first AC voltage between the heating element and a    counter electrode, and applying a DC voltage between the heating    element and the counter electrode, wherein the DC voltage is at    least 0.5 V, especially at least 1.0 V, and wherein the heating    element is chosen as positive electrode.

Optionally, one or more further AC components may be added. Below, theone or more AC components, and the DC component, are discussed in moredetail, respectively.

AC

When an alternating current (AC) is supplied to the electrodes,alternating acid and base will be formed at the electrodes. While scaleis formed during boiling it will substantially not adhere to theelectrode walls as it is constantly dissolved and re-precipitated at theelectrode surface. The method can also be used to decalcify alreadycalcified surfaces. Especially at lower frequencies the adhesion ofscale is lowered that much that it is spontaneously released from thesurface. The scale is released better the longer the calcified surfaceis in the positive mode when acid is generated. It may be that alsocorrosion of the metal can occur during this period. The sensitivity tocorrosion of the metal electrodes may be the limiting factor for thechoice of current and frequency that can be used like it is in DCdescaling. Overall however, it has been found that the loosening of thescale occurs easier then that the metal is oxidized when using ACinstead of DC. Further, higher voltages and higher currents can be usedin AC then in DC making the decalcifying effect of the electrochemicaltreatment in AC much more efficient then in DC.

Hence, the invention provides a method using a water heater arranged forheating an aqueous liquid (further indicated as “heater”) wherein theheater comprises a metal heating element to heat aqueous liquid in theheater, the method comprising:

-   a. heating aqueous liquid in the heater with the heating element,    wherein the heating element is in contact with the aqueous liquid;    and-   b. applying a first AC voltage (and a DC voltage) between the    heating element and a counter electrode.

This method can advantageously be used to (substantially) prevent scaleformation on the (surface of the) heating element, when using suchheater, but this method can also be used to remove already formed scaleon the (surface of the) heating element. The scale may neither adhere onthe (surface of the) heating element nor on the counter electrode andwill substantially remain floating in the water.

Hence, the invention further provides a method for the prevention orreduction of scale formation on a metal heating element of a waterheater arranged for heating an aqueous liquid (“heater”), the methodcomprising:

-   a. contacting the heating element with the aqueous liquid, while    preferably heating the aqueous liquid (with preferably the heating    element); and-   b. applying a first AC voltage (and a DC voltage) between the    heating element and a counter electrode.

In an especially advantageous embodiment, the first AC voltage has afirst AC frequency selected from the range of 0.1 Hz or higher, such asat least 1 Hz, such as up to 10000 Hz, like especially in the range of1-10000 Hz (i.e. 1 Hz-10 Khz), especially 5-5000 Hz, like 5-1000 Hz. Thefirst AC voltage applied may depend upon the configuration of theheating element and counter electrode, but may for instance be in therange of about 1-100 V, such as for instance 5-50 V.

Especially beneficial is the use of AC signals that have a triangularshape. In that case the time the current is at its peak is shorter thencompared to e.g. sinusoidal or block-shape waves reducing the risk ofcorrosion. In a specific embodiment, the first AC voltage has atriangular wave shape.

Application of the first AC voltage may be before, during, or afterheating of the aqueous liquid. Preferably, the first AC voltage isapplied during heating of the aqueous liquid. The phrase “applying afirst AC voltage between the heating element and a counter electrode”and similar phrases relate to the embodiment(s) that the heating elementand the counter electrode are both in contact with the aqueous liquid.Hence, the phrase “applying a first AC voltage between the heatingelement and a counter electrode” refers to “applying a first AC voltagebetween the heating element and a counter electrode, while the heatingelement and the counter electrode are in contact with the aqueousliquid. The phrase “in contact” include embodiments wherein at leastpart of the item is in contact. For instance, at least part of theheating element or at least part of the counter electrode may be incontact with the aqueous liquid, respectively.

The invention also provides an arrangement with which the method of theinvention may be applied. The invention therefore provides in anembodiment a water heater arrangement comprising a water heater arrangedfor heating an aqueous liquid, the water heater comprising a heatingelement to heat the aqueous liquid in the water heater, the heatingelement arranged to be in contact with the aqueous liquid, and anelectrical power supply, arranged to apply the first AC voltage (and aDC voltage) between the heating element and a counter electrode.

AC/DC

As mentioned above, it has surprisingly been found that further increaseof efficiency in descaling by AC can be achieved by applying anadditional DC offset to the AC. This means that the calcified surface(e.g. wall of a flow through heater or an immersed curled heatingelement) is set at a positive voltage with the counter electrodenegatively. Now, a much higher (DC) voltage can be applied when combinedwith the AC then as standalone DC. DC voltages normally not usable todecalcify the surface due to corrosion can now be used. The AC currentis tempering the corrosion effect of the DC while the scale looseningeffect adds up to the AC decalcifying.

Hence, in a further aspect, the invention provides a method of using awater heater (“heater”) arranged for heating an aqueous liquid whereinthe heater arranged for heating an aqueous liquid comprises a metalheating element to heat aqueous liquid in the heater arranged forheating an aqueous liquid, the method comprising:

-   a. heating aqueous liquid in the heater with the heating element,    wherein the heating element is in contact with the aqueous liquid;    and-   b. applying a first AC voltage between the heating element and a    counter electrode, and applying a DC voltage between the heating    element and the counter electrode, wherein the DC voltage is    preferably at least 0.5 V, more preferably at least 1.0 V,    especially at least 1.2 V, and wherein the heating element is chosen    as positive electrode.

This method can advantageously be used to (substantially) prevent scaleformation on the (surface of the) heating element, when using suchheater, but this method can also be used to remove already formed scaleon the (surface of the) heating element. The scale may neither adhere onthe (surface of the) heating element nor on the counter electrode andwill substantially remain floating in the water.

The invention thus also provides a method for the prevention orreduction of scale formation on a metal heating element of a waterheater (“heater”) arranged for heating an aqueous liquid, the methodcomprising:

-   a. contacting the heating element with the aqueous liquid, while    preferably heating the aqueous liquid (preferably with the heating    element); and-   b. applying a first AC voltage between the heating element and a    counter electrode, and applying a DC voltage between the heating    element and the counter electrode, wherein the DC voltage is    preferably at least 0.5 V, more preferably at least 1.0 V,    especially at least 1.2 V, and wherein the heating element is chosen    as positive electrode.

In general, the DC voltage will be in the range of about 1.0-10 V, suchas 1.5-10 V, especially 1.5-6 V. Further, as defined above, the first ACvoltage preferably has a first AC frequency selected from the range of0.1 Hz or higher, especially 1 Hz or higher, like up to 10000 Hz, suchas in the range of 1-10000 Hz, especially 5-1000 Hz.

Another benefit of a DC offset to an AC voltage is the following. WhenAC is applied to a system where the electrodes are of different size thecorrosion resistance of the smallest electrode may determine thecorrosion resistance of the whole setup as it has the highest currentdensity making it the most sensitive to corrosion. When for example in aflow through heater a small counter electrode is inserted to descale thewall (assuming the wall being connected to the heating element), thesmall electrode will determine the current and frequency that can beapplied. When a DC offset is applied with the right polarity, the wall(+) will benefit from the additional descaling while the small electrodeis protected against corrosion as it is connected negative (−).

Application of the first AC voltage and DC voltage may be before,during, or after heating of the aqueous liquid. Preferably, the first ACvoltage and the DC voltage are applied during heating of the aqueousliquid.

The invention also provides an arrangement with which the method of theinvention may be applied. The invention therefore provides in anembodiment a water heater arrangement comprising a water heater arrangedfor heating an aqueous liquid, the water heater comprising a heatingelement to heat the aqueous liquid in the water heater, the heatingelement arranged to be in contact with the aqueous liquid, and anelectrical power supply, arranged to apply a first AC voltage betweenthe heating element and a counter electrode, and wherein the electricalpower supply is further arranged to apply a DC voltage between theheating element and the counter electrode, wherein the DC voltage is atleast 0.5 V, and wherein the heating element is arranged as positiveelectrode.

AC/AC/DC

Although the combination of AC/DC in itself is efficient in scaleprevention the effectiveness may depend somewhat on the conductivity ofthe aqueous liquid. When the conductivity of the aqueous liquid used iscomparatively low the efficiency of the AC reduces and corrosion can setin due to the DC. To counteract this effect the frequency of the AC canbe lowered to make the AC more efficient and counteract the corrosion ofthe DC. This however may mean that AC/DC settings depend on theconductivity of the aqueous liquid. Thus a pre-measurement of theconductivity of the aqueous liquid present may be beneficial beforesetting the AC/DC parameters accordingly.

Surprisingly it has also been found that the superposition of a highfrequency AC signal on a low frequency AC signal enables the use of lowfrequency AC signals (see also above). As very low frequency signals mayespecially be efficient in descaling, the auxiliary DC offset can belowered. Overall the sensitivity to variation in aqueous liquidconductivity becomes lower making this setup more capable in handlingdifferent aqueous liquid types then single AC/DC combinations. Hence,the first AC voltage and the second AC voltage have differentfrequencies.

Hence, especially preferred is the combination of a first AC, a second(slow) AC and a DC voltage. The invention therefore also provides amethod of using a water heater (“heater”) arranged for heating anaqueous liquid wherein the heater comprises a metal heating element toheat aqueous liquid in the heater, the method comprising:

-   a. heating aqueous liquid in the heater with the heating element;    and-   b. applying a first AC voltage between the heating element and a    counter electrode, applying a second AC voltage (and optionally one    or more further AC voltages) between the heating element and the    counter electrode, wherein the second AC voltage has a second AC    frequency preferably selected from the range of 0.02 Hz-5 Hz,    especially 0.1 Hz -2.5 Hz, wherein the ratio between the first AC    frequency and the second AC frequency is preferably 2 or more, such    as in the range of 2-1000, especially at least 5, such as in the    range of 5-1000, and applying a DC voltage between the heating    element and the counter electrode, wherein the DC voltage is    preferably at least 0.5 V, more preferably at least 1.0 V,    especially at least 1.2 V, and wherein the heating element is chosen    as positive electrode.

This method can advantageously be used to (substantially) prevent scaleformation on the (surface of the) heating element, when using suchheater, but this method can also be used to remove already formed scaleon the (surface of the) heating element. The scale may neither adhere onthe (surface of the) heating element nor on the counter electrode andwill substantially remain floating in the water.

The invention thus also provides a method for the prevention orreduction of scale formation on a metal heating element of a heaterarranged for heating an aqueous liquid, the method comprising:

-   a. contacting the heating element with the aqueous liquid, while    preferably heating the aqueous liquid (preferably with the heating    element); and-   b. applying a first AC voltage between the heating element and a    counter electrode, applying a second AC voltage (and optionally one    or more further AC voltages) between the heating element and the    counter electrode, wherein the second AC voltage has a second AC    frequency preferably selected from the range of 0.02 Hz-5 Hz,    especially 0.1 Hz-2 Hz, wherein the ratio between the first AC    frequency and the second AC frequency is preferably 2 or more, such    as in the range of 2-1000, especially at least 5, such as in the    range of 5-1000, and applying a DC voltage between the heating    element and the counter electrode, wherein the DC voltage is    preferably at least 0.5 V, more preferably at least 1.0 V,    especially at least 1.2 V, and wherein the heating element is chosen    as positive electrode.

The phrase “applying a second AC voltage” and similar phrases do notexclude the application of a further (a third, . . . ) AC voltage.Hence, in a further embodiment a first AC voltage and a second ACvoltage, as defined herein, and optionally one or more further ACvoltages may be applied. Thus, in a specific embodiment, the inventionalso includes a method as defined above, further comprising applying anadditional AC voltage (in addition to the first and the second ACvoltages) between the heating element and the counter electrode.Therefore, the phrase “applying a second AC voltage (and optionally oneor more further AC voltages)” is used. For the sake of understanding,herein it is often referred to “second AC voltage”.

The second AC voltage (and optionally further AC voltages) applied maydepend upon the configuration of the heating element and counterelectrode, but may for instance also be in the range of about 1-100 V,such as for instance 5-50 V. The frequencies of the second and optionalfurther AC voltages are preferably different from each other, and mayoptionally (independently) all comply with a preferred ratio of 2 ormore of the frequency of the first AC voltage and the frequency of thesecond and further AC voltages, respectively. Further AC voltages mayhowever also have a frequency selected from the range of 0.1 Hz orhigher, especially 1 Hz or higher, like up to 10000 Hz, such as in therange of 1-10000 Hz, especially 5-1000 Hz. However, at least one set oftwo AC voltages comply with the indicated ratio range of at least 2,such as in the range of 2-1000, especially at least 5, such as in therange of 5-1000.

In a further specific embodiment, wherein at least two AC voltages areapplied, preferably both the first AC voltage and the second AC voltagehave a triangular wave shape. Optional one or more further AC voltagesmay also have a triangular wave shape.

Embodiments described above in relation to AC and AC/DC may also applyto the AC/AC/DC embodiments.

Application of the first and second AC voltages and DC voltage may bebefore, during, or after heating of the aqueous liquid. Preferably, thefirst and second AC voltages and the DC voltage are applied duringheating of the aqueous liquid.

The invention also provides an arrangement with which the method of theinvention may be applied. The invention therefore provides in anembodiment a water heater arrangement comprising a water heater arrangedfor heating an aqueous liquid, the water heater comprising a heatingelement to heat the aqueous liquid in the water heater, the heatingelement arranged to be in contact with the aqueous liquid, and anelectrical power supply, arranged to apply a first AC voltage betweenthe heating element and a counter electrode, wherein the electricalpower supply is further arranged to apply a second AC voltage (andoptionally one or more further AC voltages) between the heating elementand the counter electrode, wherein the second AC voltage has a second ACfrequency selected from the range of 0.02 Hz-5 Hz, and wherein the ratiobetween the first AC frequency and the second AC frequency is 2 or more,and wherein the electrical power supply is further arranged to apply aDC voltage between the heating element and the counter electrode,wherein the DC voltage is at least 0.5 V, and wherein the heatingelement is arranged as positive electrode.

Further Embodiments

Herein, the aqueous liquid is especially water. The method may be usedfor hard and soft water, especially for water having a waterconductivity of preferably at least 100 μS/cm.

The heating element can be immersed directly in the water or be arrangedas (part of a) wall of the heater. In both cases the heater element(wall) acts as electrode and is electrically connected to the counterelectrode. The (surface of the) heating element is thus in contact withthe aqueous liquid in the heater. This is herein also indicated by thephrase “wherein the heating element is in contact with the aqueousliquid”. Note that the term heating element thus refers to that part(element) that is in contact with the aqueous liquid and provides (whenusing the heater to heat the aqueous liquid) the heat from the heater tothe aqueous liquid. It is on the heating element (or more especially its(part of the) surface that is in contact with the aqueous liquid) thatscale may deposit. The term “heating element” may thus not necessarilyrefer to the actual heat generation device that generates the heat, butrefers to that part/element, that transfers the heat to the aqueousliquid. In an embodiment, the term “heating element” may also refer to aplurality of heating elements.

The heating element for heating the aqueous liquid herein preferablycomprises one or more metal parts for heating the liquid or isessentially from metal, such as a steel wall or a steel immersionheater. Hence, the heating element is herein also indicated as metalheating element. On this metal of the heating element that is in contactwith the aqueous liquid, scale may deposit. Preferably, the heatingelement for heating the aqueous liquid herein preferably comprises oneor more steel parts for heating the liquid or is essentially from steel.Hence, the heating element, or the part of the heating element incontact with the water is preferably made of steel. In a specificembodiment, the heating element is a steel heating element.

The term “counter electrode” may in an embodiment also refer to aplurality of counter electrodes. For instance, when more than one signalis applied, in principle different counter electrodes may be applied. Inan embodiment, the applied signals are applied on separate counterelectrodes wherein thus the counter electrode comprises a plurality ofcounter electrodes, and wherein the first AC voltage is applied betweenthe heating element and a first counter electrode, and wherein thesecond AC voltage is applied between the heating element and a furthercounter electrode and/or wherein the DC voltage is applied between theheating element and a further counter electrode. Again, the furthercounter electrode for one signal may another further counter electrodefor yet another signal. Especially when two or more AC signals applied,it may be an option to use for each AC signal a different counterelectrode. Preferably, the DC voltage is applied to each set of heatingelement and counter electrodes.

Hence, the invention also provides an embodiment of the water heaterarrangement wherein the counter electrode comprises a plurality ofcounter electrodes, and wherein the electrical power supply and theplurality of counter electrodes are arranged to apply the first ACvoltage between the heating element and a first counter electrode, andto apply the second AC voltage between the heating element and a furthercounter electrode or to apply the DC voltage between the heating elementand a further counter electrode. When both a second AC voltage and a DCvoltage are applied, different or identical further counter electrodesmay be applied. In a further embodiment, when using two or more ACvoltages and a DC voltage, the DC voltage is applied to each set ofcounter electrode and heating element, and the two or more AC voltagesare applied to the respective combinations of counter electrode andheating element.

The counter electrode may for instance be a stainless steel or a mixedmetal oxide (MMO), a carbon based or a platinum electrode. Where thewall of the heater is used as counter electrode, preferably the counterelectrode is of metal, more preferably of steel.

The term “steel” herein especially refers to stainless steel. Any gradeof stainless steel can be applied. Preferably the steel contains both Crand Ni (e.g. grade 304) while additional presence of small amounts of Mois especially beneficial (e.g. grade 316 or higher). Alternatively moreresistant metal (alloys) can be used like Inconel (Cr/Ni alloy). Thehigher the corrosion resistance of the steel the lower the frequency ofthe AC or the higher the DC offset or the voltage of the AC that can beapplied improving the scale removal.

The term “water heater” is used to indicate a device that is arranged toheat an aqueous liquid, such as water. The term “water heater” (hereinshortly indicated with “heater”) may for instance refer to a steamgeneration chamber (based on heating an aqueous liquid). The heater maybe of the flow through heater type. The heater may for instance heat theaqueous liquid in an embodiment via a heat generation device connectedto the heater wall, wherein the wall (which is in contact with theaqueous liquid), is the heating element (for heating the aqueousliquid), or may for instance in an embodiment heat via an element in theaqueous liquid, such as water, such as in the case of an immersion typeof heater (in which the heating element is in contact with the aqueousliquid), etc. Different types of heating elements may be applied (at thesame time).

The term “water heater” may also refer to a (closed) boiler arranged toproduce steam, to a (closed) boiler arranged to produce heated water, toa flow through heater or to a steamer. In a specific embodiment, theheater arranged for heating an aqueous liquid is selected from the groupconsisting of a flow through heater, a flow through steamer, a heaterfor heating water and a heater for producing steam. Included are alsoheater blocks where the heating element and e.g. the tube that carriesthe water are embedded in a block of aluminum.

Heating may be any heating at temperatures above room temperature, butespecially refers to heating (of the aqueous liquid) above 50° C., suchas especially heating the aqueous liquid in the heater to a temperatureof at least 85° C. The term heating may thus include bringing atelevated temperatures, boiling and/or producing steam.

The heater may be any heater, such as a heater of a steam generationdevice (e.g. as used for a pressurized steam generator (sometimes alsoindicated as system iron)) for providing steam, a water heater forproviding hot drinking water like in a hot liquid vending machine (e.g.for making coffee, tea, cappuccino, or hot chocolate, etc.), an electrickettle, a coffee maker (drip filter), an espresso machine, a pad coffeemachine, a boiler (for internal heating of a house (domestic boiler) orof an apartment, an office building), an industrial boiler etc.), awater heater arranged in a washing machine or in a dish washer, or ahot-water based weed killing device (or sprayer) (arranged to providinghot water to kill weed).

Hence, in a further aspect, the invention provides an electronic devicecomprising a heater arrangement wherein the electronic device isarranged to produce heated water and/or steam. Especially, theelectronic device is selected from the group consisting of an iron, apressurized steam generator, a non-pressurized steam generator(sometimes also indicated as a garment steamer), a hot liquid vendingmachine, an electric kettle, a coffee maker (drip filter), an espressomachine, a pad coffee machine, a washing machine, a dish washer, and ahot-water based weed killing device (sprayer).

As mentioned above, the herein defined voltage(s) (first AC and one ormore of a second AC and DC) are preferably applied during heating of theaqueous liquid in the heater with the heating element. This may have themost impact in preventing and/or reduction of scaling on the heatingelement.

The AC voltages may (independently) have a sinusoidal wave shape or atriangular wave shape. The wave shapes may be symmetrical orasymmetrical. Asymmetrical triangular wave shapes are sometimes alsoindicated as saw tooth wave shapes. Preferably, the wave shapes aresymmetrical. Further, as indicated above, preferably (symmetrical)triangular wave shapes are applied.

The first AC voltage, the DC voltage, and the optional further ACvoltages are applied between the heating element and the counterelectrode while the heating element and the counter electrode are incontact with the aqueous liquid. The aqueous liquid may be heated (asindicated above) (preferably with the heating element), while thevoltages are applied.

In an embodiment, the method further includes a measurement of theconductivity of the aqueous liquid, and optionally of other parameters,and optionally controlling the first AC voltage and one or more of thesecond AC voltage and the DC voltage in dependence of the measurementand predefined relations between the conductivity (and the optionalother parameters) and the first AC voltage and one or more of the secondAC voltage and the DC voltage. One or more optional other parametersthat may be measured may be selected from the group consisting of thetemperature of the aqueous liquid, the pH of the aqueous liquid, thecurrent that is running (between the heating element and the counterelectrode), the voltage drop when connecting the two electrodes (i.e.the heating element and the counter electrode), etc.

Typically, the current density (i.e. between the heating element and thecounter electrode) is in the range of 0.1-10 mA/cm², especially 0.1-5mA/cm², such as especially 0.2-0.6 mA/cm², when using a flat heatingelement or a spiral shaped heating element in a boiler system, orespecially 0.2-5 mA/cm² for a flow through heater.

The electrical power supply can be any system that is able to generate afirst AC voltage and one or more of a second AC voltage and a DC voltageoffset. Optionally, one or more of the frequency of the first AC, thepeak to peak voltage of the first AC, the frequency of the second AC (ifapplicable), the peak to peak voltage of the second AC (if applicable),and the voltage of the DC, are variable and controllable, for instanceone or more may be controlled in relation to a parameter like electricconductivity of the liquid and/or temperature of the liquid, or thecurrent that is running. The term electrical power supply may in anembodiment also refer to a plurality of electrical power supplies. Inprinciple, each voltage may be generated by a different electrical powersupply.

Hence, the invention especially provides the use of a combination of afirst AC voltage and one or more of a second AC voltage (and optionallyone or more further AC voltages) and a DC voltage to a metal heatingelement of a heater arranged for heating an aqueous liquid (and arrangedto be in contact with the aqueous liquid), to prevent or reduce scalingon the metal heating element, especially during heating of the aqueousliquid with the heating element in the heater. In a specific embodiment,both AC voltages and the DC voltage are applied.

The application of the voltage(s) may be applied preferably permanentlyduring the time the aqueous liquid is at elevated temperatures, but mayin an embodiment also be applied periodically. Optionally, thevoltage(s) are applied before or after heating of the aqueous liquid.However, best results are obtained when the voltage(s) are applied atleast during heating of the aqueous liquid.

Preferred conditions with respect to the frequencies and voltage(s) areindicated in the table below:

1^(st) AC + DC + optionally 2^(nd) AC Ratio 1^(st) AC frequency/2^(nd)AC 1^(st) AC DC 2^(nd) AC frequency Applied ≥1 Hz; n.a. 0.02 Hz-5 Hz,2-1000; frequency especially especially especially 5-500 Hz 0.1 Hz-2 Hz5-500. voltage 1.0-10 V; especially 1.2-6 V

Other preferred conditions are indicated in the next table below:

Water conductivity Water temperature 100-50,000 μS/cm 50° C.-boilingtemperature; especially ≥85° C.

The term “substantially” herein, will be understood by the personskilled in the art. The term “substantially” may also includeembodiments with “entirely”, “completely”, “all”, etc. Hence, inembodiments the adjective substantially may also be removed. Whereapplicable, the term “substantially” may also relate to 90% or higher,such as 95% or higher, especially 99% or higher, even more especially99.5% or higher, including 100%. The term “comprise” includes alsoembodiments wherein the term “comprises” means “consists of”.

Furthermore, the terms first, second, third and the like in thedescription and in the claims, are used for distinguishing betweensimilar elements and not necessarily for describing a sequential orchronological order. It is to be understood that the terms so used areinterchangeable under appropriate circumstances and that the embodimentsof the invention described herein are capable of operation in othersequences than described or illustrated herein.

The devices (such as the heater arrangement) herein are amongst othersdescribed during operation (i.e. especially heating the aqueous liquid).As will be clear to the person skilled in the art, the invention is notlimited to methods of operation or devices in operation.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.Use of the verb “to comprise” and its conjugations does not exclude thepresence of elements or steps other than those stated in a claim. Thearticle “a” or “an” preceding an element does not exclude the presenceof a plurality of such elements. The invention may be implemented bymeans of hardware comprising several distinct elements, and by means ofa suitably programmed computer. In the device claim enumerating severalmeans, several of these means may be embodied by one and the same itemof hardware. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIGS. 1a-1f schematically depict some possible configurations of aheater arrangement where the heating element is immersed in the water orwhere the heating element is a wall of a water heater;

FIGS. 2a-2g schematically depict some examples of DC and AC voltagesthat may be applied; and

FIG. 3 schematically depicts an embodiment of an electronic devicecomprising the heater arrangement.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1a schematically depicts a water heater arrangement (“heaterarrangement”) 1 comprising a water heater (“heater”) 100, arranged forheating an aqueous liquid 20. The aqueous liquid 20, especially water,is contained in the heater 100. The heater 100 comprises a metal heatingelement 110 to heat the aqueous liquid 20 in the heater 100. The heaterarrangement 1 further comprises an electrical power 200 supply, arrangedto apply a first AC voltage between the heating element 110 and acounter electrode 120. In addition to the first AC voltage a second,lower frequency AC voltage may be added and/or a DC voltage. When a DCvoltage is applied, the heating element 110 is chosen as positiveelectrode. By application of the voltage(s), the metal heating element110 is protected against scaling and/or scaling formed may be removed.FIG. 1a schematically depicts an embodiment wherein the heating element110 is a submerging/immersion heating element, indicated with reference111. The wall (or at least part of it) of the heater 100 is used ascounter electrode in this embodiment. The wall may for instance be ofsteel as conductive material. Typically stainless steel is used with Cr,Ni as alloying elements (e.g. 304) or with additional Mo added (e.g.316). Alternatively more resistant metal (alloys) can be used likeInconel (Cr/Ni alloy).

FIG. 1b schematically depicts a heater arrangement 1 wherein the heatingelement 110 is a curled immersed heating element, and wherein thecounter electrode 120 is partly surrounded by at least part of thesubmerging heating element 111. In this case the counter electrode canbe of stainless steel or Inconel or other oxidation resistant electrodematerial like titanium, platinum, mixed metal oxide coated titanium,platinum coated titanium or carbon based.

FIG. 1c schematically depicts an embodiment of a flow through heater(FTH), wherein the heater 100 is a heater through which the aqueousliquid 20 flows, while being heated. In the schematic embodiment of FIG.1c , a heat generation device 115 is connected to the wall of the heater100, and a rod within the heater is used as counter electrode 120. Thewall is connected to a generation device 115 to heat the wall and ispreferably of (stainless) steel; the wall is in contact with the aqueousliquid (not depicted) and is thus used as heating element 110. Thecounter electrode 120 may comprise a material as indicated in thedescription above for FIG. 1 b.

FIG. 1d schematically depicts substantially the same embodiment asschematically depicted in FIG. 1c , but now in cross-sectional view.Elements 115 heat wall of the heater 100. Therefore, the wall isindicated as heating element 110. Over this wall, i.e. heating element110, and the counter electrode 120, a voltage is applied with electricalpower supply 200. Here, the wall is used as heating element 110, and ispreferably of (stainless) steel. The counter electrode 120 may comprisea material as indicated in the description above for FIG. 1 b.

FIG. 1e schematically depicts substantially the same embodiment asschematically depicted in FIG. 1a , with the difference that two counterelectrodes are applied, indicated with references 120 a and 120 b. Here,the wall is used as counter electrode 120, and thus comprises two (ormore) isolated parts that are used as counter electrodes. For instance,when a first AC and a second AC are imposed over the counterelectrode(s) and the heating element 110, the first AC may be appliedover the heating element 110 and the first counter electrode, indicatedwith reference 120 a, and the second AC may be applied over the heatingelement 110 and the second or further counter electrode, indicated withreference 120 a.

FIG. 1f schematically depicts substantially the same embodiment asschematically depicted in FIG. 1b . Here, also two counter electrodes(120) are used (like in schematic FIG. 1e ), indicated with references120 a and 120 b. As indicated above, when for instance a first AC and asecond AC are imposed over the counter electrode(s) and the heatingelement 110, the first AC may be applied over the heating element 110and the first counter electrode, indicated with reference 120 a, and thesecond AC may be applied over the heating element 110 and the second orfurther counter electrode, indicated with reference 120 a.

The voltages applied with the electrical power supply (or powersupplies) may be AC/DC and optional further AC components, and AC/AC/DCand optional further AC components.

FIG. 2a schematically depicts an AC voltage signal, with time on thex-axis and the voltage on the y-axis. The peak to peak voltage is 10units in this schematic figure. FIG. 2b schematically depicts the sameAC voltage signal, but now with a DC voltage superimposed on the ACvoltage (i.e. AC/DC). The DC voltage is negative, and the whole signalshifts to the negative. Preferably, the heating element is chosen aspositive electrode, and the counter electrode is chosen as negativeelectrode (with respect to the application of the DC voltage). FIG. 2cschematically depicts an AC/AC signal, with a fast component and a slowcomponent. FIG. 2d schematically depicts the same AC/AC signal as inFIG. 2c , but now with a DC voltage superimposed on the AC voltage (i.e.AC/AC/DC). The DC voltage is negative, and the whole signal shifts tothe negative. Here, by way of example sinusoidal AC voltages areapplied. Preferably, the AC voltages have a triangular wave shape, thuswith substantially flat ramps. FIG. 2e schematically depicts suchsignal. In FIG. 2e the signal is symmetric. This however does notpreclude the use of asymmetric signals (with non identical slopes).Superimposed thereon can a second (slower) AC and/or a DC voltage beapplied. The second AC may also have a triangular wave shape. FIG. 2fschematically depicts AC/AC voltages both having a triangular waveshape, and FIG. 2g schematically depicts the same, but now including aDC offset.

FIG. 3 schematically depicts an electronic device 2. FIG. 3schematically depicts an electric kettle as example of the electronicdevice 2. The electronic device 2 comprises the heater arrangement 1.Here, electronics 300 may be arranged to control the heating of heatingelement 120 and provide power to the electric power supply 200 forimposing the first AC and optional second AC and/or optional DC voltageto the heating element 120 and counter electrode 110.

The heater arrangement 1 may further comprise a sensor (not depicted) tosense parameters like conductivity of the aqueous liquid, thetemperature of the aqueous liquid, etc. Further, the heater arrangement1 may further comprise a controller, to control the one or more featuresof the first AC and one more features of the one or more of the secondAC and DC. The controller may control those one or more features independence of the one or more parameters and one or more predefinedrelations between the one or more parameters and the one or morefeatures.

EXAMPLES

Water Preparation

Stock solutions of CaCl₂.2H₂O (65.6 gr/ltr), MgSO₄.7H₂O (38 gr/ltr) andNaHCO₃ (76.2 gr/ltr) were made. Standard hard water was made by mixing50 gram of each stock solution into 9 liter of de-ionized water andadding up to 10 liter. The resulting water had a total hardness of(around) 16.8° DH (German degree of hardness) and a temporary hardnessof (around) 11.2° DH.

Total hardness is defined as 2.8×2×[mmol/ltr Ca+mmol Mg/ltr]; Temporaryhardness is defined as 2.8×[mmol HCO₃ ⁻/ltr].

Calcified Electrode Preparation

In a typical experiment hard water was added to a 250 ml beaker glass. Acurled heating element (of stainless steel) was immersed into the water.Water was boiled and kept at 95° C. for 30 minutes. During heating astainless steel tube of 12 mm diameter and a 1 mm Metal oxide (MMO)coated Titanium electrode were immersed in the water at 5 cm depth. Thedistance between the electrodes was 1 cm. Both electrodes wereelectrically connected at 4.5 V DC with the stainless steel set asnegative electrode. Scale deposited on the tube and adhered firmly.

Descaling Experiments

For every experiment a freshly calcified electrode (the stainless steeltube) was used. For the experiments the same setup was used as forcalcifying the test electrode. After boiling for 30 minutes theelectrode (the stainless steel tube) was investigated for spontaneousremoval of the scale and for corrosion. After that the electrode (thestainless steel tube) was rinsed under running tap water with gentlerubbing to further test the loosening of the scale.

Experiment 1 (AC) (Reference Example)

A calcified 12 mm stainless steel tube and a small strip of MMO coatedtitanium of 6 mm width (the counter electrode) were connected to a powersupply. 6.8 Vpp was applied with a frequency of 100 Hz resulting in 40mA current. (As power source a function generator was used with 50 Ohmimpedance). After heating for 30 minutes at 95° C. the outside of thetube (the electrode) was checked. No spontaneous scale removal wasobserved. Rinsing under tap water with gentle rubbing showed poorremoval of scale. No corrosion was observed.

Experiment 2 (AC) (Reference Example)

A calcified 12 mm stainless steel tube and a small strip of MMO coatedtitanium of 6 mm width were connected to a power supply. 6.8 Vpp wasapplied with a frequency of 10 Hz resulting in 40 mA current. Afterheating for 30 minutes at 95° C. the electrode was checked. Nospontaneous scale removal was observed. Rinsing under tap water withgentle rubbing showed good removal of scale. No corrosion was observed.

Experiment 3 (AC) (Reference Example)

A calcified 12 mm stainless steel tube and a small strip of MMO coatedtitanium of 6 mm width were connected to a power supply. 6.8 Vpp wasapplied with a frequency of 5 Hz resulting in 40 mA current. Afterheating for 30 minutes at 95° C. the electrode was checked. Scale hadloosened from the metal tube spontaneously. Rinsing under tap water withgentle rubbing showed very good removal of scale beyond the spontaneouscleaned area. No corrosion was observed.

Experiment 4 (AC) (Reference Example)

A calcified 12 mm stainless steel tube and a small strip of MMO coatedtitanium of 6 mm width were connected to a power supply. 6.8 Vpp wasapplied with a frequency of 2.5 Hz resulting in 40 mA current. Afterheating for 30 minutes at 95° C. the electrode was checked. Scale hadloosened from the metal tube spontaneously Rinsing under tap water withgentle rubbing showed very good removal of scale beyond the spontaneouscleaned area. A slight yellow color was observed on the tube indicatingthe beginning of corrosion.

When the frequency is lowered too much corrosion of the steel electrodemay occur suggesting that lower current may be beneficial.

Experiment 5 (AC) (Reference Example)

A calcified 12 mm stainless steel tube and a small strip of MMO coatedtitanium of 6 mm width were connected to a power supply. 4.8 Vpp wasapplied with a frequency of 2.5 Hz resulting in 20 mA current. Afterheating for 30 minutes at 95° C. the electrode was checked. Scale hadloosened from the metal tube spontaneously Rinsing under tap water withgentle rubbing showed good removal of scale beyond the spontaneouscleaned area. No corrosion was observed.

To evaluate the effect of a DC current the following experiments 6-8were performed.

Experiment 6 (DC) (Reference Example)

A calcified 12 mm stainless steel tube and a small strip of MMO coatedtitanium of 6 mm width were connected to a power supply. 0.3 V DC wasapplied resulting in 0.1 mA current. After heating for 30 minutes at 95°C. the electrode was checked. Rinsing under tap water with gentlerubbing showed poor removal of scale. There was no spontaneous removalof scale. No corrosion was observed.

In DC descaling experiments, the stainless steel tube is used aspositive electrode and the counter electrode, the MOx coated titanium,as negative electrode.

Experiment 7 (DC) (Reference Example)

A calcified 12 mm stainless steel tube and a small strip of MMO coatedtitanium of 6 mm width were connected to a power supply. 0.6 V DC wasapplied resulting in 0.3 mA current. After heating for 30 minutes at 95°C. the electrode was checked. Rinsing under tap water with gentlerubbing showed limited removal of scale. There was no spontaneousremoval of scale. No corrosion was observed.

Experiment 8 (DC) (Reference Example)

A calcified 12 mm stainless steel tube and a small strip of MMO coatedtitanium of 6 mm width were connected to a power supply. 1.0 V DC wasapplied, with the stainless steel tube as positive electrode.Immediately pitting corrosion was observed at the stainless steel tube.After 2 minutes the experiment was stopped. Rinsing under tap water withgentle rubbing showed poor removal of scale. There was no spontaneousremoval of scale.

Higher voltages were not pursued. The poor efficiency in decalcificationand the sensitivity to corrosion of DC only are clearly demonstrated inexperiment 6-8.

Experiment 9 (AC/DC)

A calcified 12 mm stainless steel tube and a small strip of MMO coatedtitanium of 6 mm width were connected to a power supply. 4.8 Vpp AC wasapplied with a frequency of 5 Hz resulting in 20 mA current. 1.0 V DCwas applied as offset with the tube being the positive electrode. Afterheating for 30 minutes at 95° C. the electrode was checked. Scale hadloosened from the metal tube spontaneously. Rinsing under tap water withgentle rubbing showed very good removal of scale beyond the spontaneouscleaned area. No corrosion was observed.

Experiment 10 (AC/DC)

A calcified 12 mm stainless steel tube and a small strip of MMO coatedtitanium of 6 mm width were connected to a power supply. 4.8 Vpp AC wasapplied with a frequency of 5 Hz resulting in 20 mA current. 1.5 V DCwas applied as offset with the tube being the positive electrode. Afterheating for 30 minutes at 95° C. the electrode was checked. Scale hadloosened from the metal tube spontaneously Rinsing under tap water withgentle rubbing showed very good removal of scale beyond the spontaneouscleaned area. No corrosion was observed.

The corrosion tempering effect of a AC on a relative high DC is clearfrom experiments 9 and 10.

Experiments 11 and 12 show the benefit of a DC offset in protectingsmall counter electrodes and loosening the scale

Experiment 11 (AC) (Reference Example)

A calcified 12 mm stainless steel tube and a stainless steel rod of 1 mmwidth were connected to a power supply. 4.8 Vpp AC was applied with afrequency of 5 Hz resulting in 20 mA current. After heating for 30minutes at 95° C. the electrode was checked. Scale did not loosenspontaneously from the metal tube. Rinsing under tap water with gentlerubbing showed good removal of scale. The 1 mm diameter counterelectrode however showed signs of corrosion.

Experiment 12 (AC/DC)

A calcified 12 mm stainless steel tube and a stainless steel rod of 1 mmwidth were connected to a power supply. 4.8 Vpp AC was applied with afrequency of 5 Hz resulting in 20 mA current. 1.5 V DC was applied asoffset. After heating for 30 minutes at 95° C. the electrode waschecked. Scale had loosened from the metal tube spontaneously. Rinsingunder tap water with gentle rubbing showed very good removal of scalebeyond the spontaneous cleaned area. No corrosion was observed on boththe pre-calcified tube and the small counter electrode.

Calcified Electrode Preparation

In a typical experiment hard water was added to a 600 ml beaker glass. Acurled heating element was immersed into the water. Water was boiled andkept at 95 C for 30 minutes. During heating a stainless steel tube of 12mm diameter and 10 mm inner diameter was immersed into the water. A 1 mmMetal oxide (Mox) coated Titanium electrode was positioned in the centerof the tube. Both electrodes were electrically connected at 3.5 V DCwith the stainless steel set as negative electrode. Scale deposited onthe inside of the tube and adhered firmly.

Experiments 13 and 14 show the influence of water conductivity ondescaling and corrosion behavior at very low frequency in combination ofan offset. Experiments 15 and 16 show the benefit of additional ACsignal on prevention of corrosion.

Experiment 13 (AC/DC)

A calcified (inside) 12 mm stainless steel tube and a 6 mm diameter rodof stainless steel centered inside were connected to a power supplyafter immersion in standard hard water of 9000/cm conductivity. At 95°C. a current of 40 mA was applied at a measured 2.6 Vpp, with afrequency of 0.5 Hz and an offset of −1.5V DC (with the stainless steeltube as positive electrode and the rod as negative counter electrode).After heating for 30 minutes the tube and the rod were checked. Allscale had been removed although some signs of corrosion were observed.

Experiment 14 (AC/DC)

The same experiment as experiment 12 was repeated in water of 300 μS/cmconductivity (water with relatively low hardness). A voltage of 3.8 Vwas applied to obtain the same current. After heating for 30 minutes at95° C. the tube and the rod were checked. All scale had been removed andno corrosion was detected.

Experiment 15 9AC/AC/DC)

A calcified 12 mm stainless steel tube and a 6 mm diameter rod ofstainless steel centered inside were connected to a power supply afterimmersion in water of 900 μS conductivity. A current of 40 mA wasapplied with a frequency of 0.5 Hz and an offset of −1.5V DC.Superimposed onto the signal was added a signal of the same amplitudebut with zero offset and a frequency of 2.5 Hz. After heating for 30minutes at 95° C. the tube and the rod were checked. All scale had beenremoved and no corrosion was detected showing the benefit of anadditional AC current.

Experiment 16 (AC/AC/DC)

A calcified 12 mm stainless steel tube and a 6 mm diameter rod ofstainless steel centered inside were connected to a power supply afterimmersion in water of 900 μS conductivity. At 95° C. a current of 40 mAwas applied with a frequency of 0.5 Hz and an offset of −1.5V DC.Superimposed onto the signal was added a signal of the same amplitudebut with zero offset and a frequency of 1000 Hz. After heating for 30minutes at 95° C. the tube and the rod were checked. All scale had beenremoved and no corrosion was detected showing the benefit of anadditional AC current.

In the following experiments the curled heating element was used aselectrode.

Calcified Heating Element Preparation

In a typical experiment hard water was added to a 600 ml beaker glass. Acurled heating element (stainless steel) was immersed into the water toheat the water. Water was kept at 95° C. for 30 minutes. During heatingan L-shape MOX coated Ti electrode was positioned inside the heatingelement. The heating element and the counter electrode were connected toa power supply at 3.5 V DC with the element connected as the negativeelectrode. Scale deposited on the heating element and adhered firmly.

Experiment 17 (AC/AC/DC)

A calcified heating element was immersed in standard hard water of 900μS. An L-shape stainless steel electrode of 6 mm diameter was positionedinside the element. A current of 40 mA was applied to the system using 2ramp shape AC signals of respectively 0.25 and 2.5 Hz and an offset of−2.5 V DC (with the curled heating element as positive electrode and theL-shape stainless steel electrode as negative counter electrode). Afterheating for 30 min the element and electrode were checked. Scale hadbeen removed and no corrosion was detected. Same results were obtainedwhen the offset was increased up to −4.0 V DC.

Experiment 18 (AC/AC/DC)

In the middle of a flow through heater tube with length of 18 cm andinner diameter of 12 mm was positioned a mixed metal oxide (MMO) coatedtitanium rod of 2 mm diameter. During 30 minutes hard water was pumpedthrough the heater at 140 ml/min. The water left the tube at atemperature of 95° C. A DC voltage of 3.5 V was applied during theheating with the heater as negative electrode and the counter electrodeas the positive electrode. Scale deposited on the wall of the heater.

After the calcification two superimposed frequencies of 0.1 and 1 Hzwere applied. The offset was 2.5 V with the heater as positiveelectrode. A current of 130 mA was measured giving a current density ofaround 2 mA/cm² on the wall of the heater. After pumping and heatinghard water again for 30 min the tube was observed to have beendecalcified. At 190 mA (2.8 mA/cm²) similar results were observed.

Experiment 19 (AC/AC/DC)

A stainless steel cylindrical shaped cup with a heating elementconnected at the outside of the flat bottom was filled with hard water.The radius of the bottom was 5.25 cm. At 3 mm distance from the bottom astainless steel spiral electrode of 2 mm wire thickness was positioned.The water was heated while two super imposed AC signals (triangularshape) of 1 and 1000 Hz were applied. An offset of 1.5 V was used withthe cup as positive electrode. A current of 78 mA was measured giving acurrent density taking only the bottom into account of 0.9 mA/cm².Taking the whole cup into account with a water level of 3 cm the currentdensity on the wall of the cup was 0.4 mA/cm². After 20 minutes heatingthe cup was emptied. No scale had adhered to the wall. At 56 mA (0.3mA/cm²) similar results were obtained.

The invention claimed is:
 1. A method of using a water heater arrangedfor heating an aqueous liquid wherein the water heater comprises aheating element to heat aqueous liquid in the water heater, the methodcomprising: a. heating aqueous liquid in the water heater with theheating element, wherein the heating element is in contact with theaqueous liquid; and b. applying a first AC voltage between the heatingelement and a counter electrode, and applying a DC voltage between theheating element and the counter electrode, wherein the DC voltage is atleast 0.5 V, and wherein the heating element is chosen as a positiveelectrode.
 2. The method according to claim 1, wherein the first ACvoltage has a first AC frequency of at least 0.1 Hz.
 3. The methodaccording to claim 1, further comprising applying a second AC voltage,wherein the second AC voltage comprises one or more further AC voltagesin addition to the first AC voltage, between the heating element and thecounter electrode, wherein the second AC voltage has a second ACfrequency selected from the range of 0.02 Hz-5 Hz and wherein the ratiobetween the first AC frequency and the second AC frequency is 2 or more.4. The method according to claim 3, wherein the counter electrodecomprises a plurality of counter electrodes, and wherein the first ACvoltage is applied between the heating element and a first counterelectrode, and wherein the DC voltage is applied between the heatingelement and a further counter electrode or wherein the second AC voltageis applied between the heating element and the further counterelectrode.
 5. The method according to claim 4, wherein at least one ofthe first AC voltage, the DC voltage, and the second AC voltage isapplied during heating of the aqueous liquid in the water heater withthe heating element.
 6. The method according to claim 1, comprisingheating aqueous liquid in the water heater to a temperature of at least85 ° C.
 7. The method according to claim 1, wherein the first AC voltagehas a triangular wave shape, and wherein the triangular wave shape issymmetrical.
 8. The method according to claim 3, wherein the first ACvoltage has a triangular wave shape, and wherein the second AC voltagealso has a triangular wave shape, and wherein the triangular wave shapesare symmetrical.
 9. The method according to claim 3, wherein the methodfurther includes a measurement of a conductivity of the aqueous liquid,and controlling the first AC voltage and one or more of (i) the secondAC voltage and (ii) the DC voltage in dependence of the measurement andpredefined relations between the conductivity, and the first AC voltageand one or more of (i) the second AC voltage and (ii) the DC voltage.10. The method according to claim 3, wherein the method further includesa measurement of a conductivity of the aqueous liquid and of otherparameters selected from the group consisting of a temperature of theaqueous liquid, a pH of the aqueous liquid, a current that is runningbetween the heating element and the counter electrode, a voltage drop inresponse to connecting the heating element and the counter electrode,and controlling the first AC voltage and one or more of (i) the secondAC voltage and (ii) the DC voltage in dependence of the measurement andpredefined relations between the conductivity, and the other parameters,and the first AC voltage and one or more of (i) the second AC voltageand (ii) the DC voltage.
 11. A water heater arrangement comprising: aheating element to heat an aqueous liquid in a water container, theheating element arranged to be in contact with the aqueous liquid, andan electrical power supply, arranged to apply a first AC voltage betweenthe heating element and a counter electrode, wherein the electricalpower supply is further arranged to apply a DC voltage between theheating element and the counter electrode, wherein the DC voltage is atleast 0.5 V, and wherein the heating element is arranged as a positiveelectrode.
 12. The water heater arrangement according to claim 11,wherein the first AC voltage has a first AC frequency of at least 1 Hz.13. The water heater arrangement according to claim 11, wherein theelectrical power supply is further arranged to apply a second ACvoltage, wherein the second AC voltage comprises one or more further ACvoltages in addition to the first AC voltage, between the heatingelement and the counter electrode, wherein the second AC voltage has asecond AC frequency selected from the range of 0.02 Hz -5 Hz, andwherein the ratio between the first AC frequency and the second ACfrequency is 2 or more.
 14. The water heater arrangement according toclaim 11, wherein the counter electrode comprises a plurality of counterelectrodes, and wherein the electrical power supply and the plurality ofcounter electrodes are arranged to apply the first AC voltage betweenthe heating element and a first counter electrode, and to apply the DCvoltage between the heating element and a further counter electrode orto apply the second AC voltage between the heating element and a furthercounter electrode.
 15. An electronic device comprising a water heaterarrangement according to claim 11, wherein the electronic device isarranged to produce one or more of heated water and steam.
 16. Theelectronic device according to claim 15, wherein the electronic deviceis selected from the group consisting of an iron, a pressurized steamgenerator, a non-pressurized steam generator, a hot liquid vendingmachine, an electric kettle, a coffee maker, an espresso machine, awashing machine, a dish washer, and a hot-water based weed killingdevice.
 17. The method of claim 1, further comprising: using acombination of (i) the first AC voltage, (ii) the DC voltage, and (iii)one or more second AC voltage applied to the heating element of thewater heater arranged for heating the aqueous liquid, wherein thecombination is operable to prevent or reduce scaling of the waterheater.